MAX868CSLASHD datasheet(8/12 Pages) MAXIM

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Regulated, Adjustable -2x

Inverting Charge Pump

___________________________________

__________Applications Information

Low-Output-Voltage Operation

Since the difference between the voltage of the series-

connected flying capacitors and the output voltage

must be dissipated within the device, the MAX868’s

efficiency is very similar to that of a linear regulator.

Estimate efficiency using the following equation:

where k is a constant equal to 2 for the standard con-

figuration of Figure 5 and equal to 1 for the circuit of

Figure 6. This equation’s denominator is the voltage

resulting from the series connection of the flying capac-

itors (-2 x VIN, as shown in Figure 3b), while its numera-

tor is simply the regulated output voltage.

For applications in which the output voltage will not be

more negative than -

|VIN|, the efficiency can be doubled

using the circuit of Figure 6, as compared to the circuit

of Figure 5. In Figure 6, a single flying capacitor is con-

nected between C2+ and C1-, with C2- and C1+ left

unconnected. Furthermore, doubling the flying capaci-

tor to provide the same flying capacitance as the stan-

dard configuration (i.e., setting CF = C1 + C2) provides

the same load-current capability as the standard con-

figuration and reduces the MAX868’s open-loop output

resistance by a factor of two, due to the reduction in the

number of switches in the current path.

Layout and Grounding

Proper layout is important to obtain optimal perfor-

mance. Connect GND to PGND together using the

shortest trace possible, and similarly connect these

pins to the ground plane. Mount all capacitors as close

to the MAX868 as possible, keeping traces short to

minimize parasitics. Keep all connections to the FB pin

as short as possible. Specifically, locate R1 and R2

next to FB (Figures 7 and 8). Should it become neces-

sary in the final layout, leave room to parallel a feed-

forward capacitor across R1.

k x V

OUT

MAX868

C1+

PGND

SHDN

GND

500k

750k

OUT

0.1

µF

VOUT = -7.5V

VIN = 5V

0.1

µF

C1-

C2+

C2-

Figure 5. Standard Configuration for Generating an Output

Voltage up to -2 x VIN

MAX868

C2+

PGND

SHDN

GND

OUT

CF = 0.2µF

µF

VOUT = -3.3V

AT 20mA

VIN = 5V

C2-

C1+

C1-

*C1+ AND C2- MUST BE LEFT UNCONNECTED.

500k

330k

Figure 6. Alternative Configuration for

|VOUT| ≤ VIN

Chip Information

TRANSISTOR COUNT: 96

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